The present invention relates to methods for reducing the levels of oxidized chromium ion, particularly Cr+6, in drinking or wastewater.
Currently, the United States Environmental Protection Agency (USEPA) allows up to 100 parts per billion (ppb) total chromium (including chromium in any of its predominant oxidative states, including Cr+2, Cr+3, Cr+4, Cr+5 and Cr+6) in drinking water. Many states have considerably lower allowances; for example, the state standard in California is 50 ppb total chromium. The USEPA currently allows up to 2.77 parts per million (ppm) daily and 1.30 ppm as a monthly average chromium ion in discharge wastewater. A growing concern about the carcinogenicity of Cr+6 is prompting some states to consider dramatically reducing the amount of Cr+6 allowed in drinking and wastewater. California, for example, recently signed into law a requirement for the Department of Health Services to adopt a Cr+6 MCL by Jan. 1, 2004. A standard of less than 1 ppb Cr+6 in drinking water has been considered in some states. Additionally, the USEPA is considering lowering the allowable chromium ion in discharge by approximately 50%.
Since many water sources, such as reservoir and well sources, contain levels of Cr+6 considerably above these levels, the proposed new standards would force many water treatment facilities to further treat water to remove Cr+6 levels. The USEPA has currently approved Coagulation/Filtration, Ion Exchange, Reverse Osmosis, and Lime Softening treatment methods for removing chromium from drinking water.
Accordingly, it is an object of the invention to provide a simple, inexpensive method for treating water to substantially remove or greatly reduce the amount of an undesirable chromium ion, such as Cr+6, present in drinking water.
It is another object of the invention to provide a treatment technology, which enables effluent to be polished or further treated to substantially remove or greatly reduce the amount of an undesirable chromium ion, such as Cr+6, present in the effluent.
In one aspect, the invention includes a method for removing or substantially reducing the amount of oxidized chromium ion present in water supplied from the plant as drinking water. Specifically, the method is useful for treating water in conjunction with a water treatment method that includes the steps of obtaining water at a wellhead, transporting the water to a water-treatment plant, and treating the water at the plant with an oxidant. In the method, prior to exposing the water to the oxidant, an amount of a stannous salt of an inorganic acid or an organic acid effective to reduce a substantial portion of any Cr+6 present to a lower oxidation state is added.
In one embodiment, the amount of a stannous salt of an inorganic acid or an organic acid is added that is effective to reduce any Cr+6 present to a lower oxidation state such that the final Cr+6 concentration is less than or equal to 100 ppb. In another embodiment, the final Cr+6 concentration is less than or equal to 50 ppb. In yet another embodiment, the final Cr+6 concentration is less than or equal to 1 ppb.
In one embodiment, the step of adding includes (i) determining the level of Cr+6 in the water obtained from the wellhead, and (ii) adding an excess stoichiometric amount of Sn+2 to the water, where the stoichiometric amount is defined as 3Sn+2 to 2Cr+6.
In another embodiment, the excess stoichiometric added is about a 1.1-10 fold excess amount of Sn+2.
The stannous salt is added in an amount of between about 50-200 parts Sn+2 per billion on a weight basis, in another embodiment. The stannous salt is, in one embodiment, SnCl2. The stannous salt can be added, for example, in an acidified aqueous slurry or solution of stannous salt.
The stannous salt is added, in another embodiment, as an acidified composition consisting essentially of SnCl2, water and HCl.
In one embodiment, the lower oxidation state of Cr+6 is Cr+3.
It will be appreciated that in another embodiment, the water can be obtained at a wellhead and transported directly to a residence.
In another aspect, the invention includes a method of treating wastewater by removing or substantially reducing the amount of oxidized chromium ion by treating the water with an oxidant; and adding to water, after to exposing the water to the oxidant, an amount of a stannous salt of an inorganic acid effective to reduce a substantial portion of any Cr+6 present to a lower oxidation state.
In one embodiment, the step of adding includes (i) determining the level of Cr+6 in the water, and (ii) adding an excess stoichiometric amount of Sn+2 to the water, where the stoichiometric amount is defined as 3Sn+2 to 2Cr+6.
The excess stoichiometric amount is, in one embodiment, a 1.1-10 fold excess amount of Sn+2.
In another embodiment, the stannous salt is added in an amount of between about 50-200 parts Sn+2 per billion on a weight basis. The stannous salt is, in one embodiment, SnCl2.
The stannous salt is added, in another embodiment, as an acidified composition consisting essentially of SnCl2, water and HCl.
The method further includes, in another embodiment, filtrating the water after addition of the stannous salt.
In one embodiment, the lower oxidation state of Cr+6 is Cr+3.
The present invention is designed for use in a water treatment method for treating wastewater or drinking water that includes the steps of adding to the water an amount of a stannous salt of an inorganic acid or organic acid effective to reduce a substantial portion of any Cr+6 present in the water to lower oxidation states of chromium, as represented by Cr+3.
These and other objects and features of the invention will become more fully apparent when the following detailed description of the invention is read in conjunction with the accompanying drawings.